1. Field of the Invention
The present invention relates to a manufacturing method of a color wheel suitable for use as a filter element of a time-share light dispersing device, and to a color wheel fabricated by the manufacturing method and incorporated in a color wheel assembly making up a projection-type image display apparatus.
2. Description of the Related Art
Color composition in a projection-type image display apparatus has conventionally been accomplished commonly by a method, such as: a single-panel method, in which one light valve element adapted to control light amount per pixel thereby creating an image is used to disperse each pixel into red (R), green (G), and blue (B) lights; and a three-panel method, in which three light valve elements dedicated to R, G and B lights are used to produce in parallel R, G and B images, and then the three images thus produced are composed. Recently, as a light valve element capable of fast switching, such as a ferroelectric liquid crystal display element or a digital micro mirror device, is increasingly coming into practical use, a time-sharing single-panel method is widely used. In the time-sharing single-panel method, R, G and B lights are caused to sequentially impinge on one light valve element, the light valve element is driven in synchronization with switching-over of the R, G and B lights thereby producing R, G and B images in a time-series manner, and the images thus produced are projected onto a screen, or the like. Here, color composition of the images is accomplished by a viewer due to an afterimage effect occurring at a sense of vision. In the time-sharing single-panel method, reduction in both dimension and weight of the apparatus, which is a feature of a single-panel method, can be achieved by employing a relatively simple optical system, and therefore the time-sharing single-panel method is favorable for realizing inexpensive fabrication of a projection-type image display apparatus. In such an image display apparatus, a color wheel is preferably used as a filter element of a time-share light dispersing device to sequentially disperse light emitted from a white light source into R, G and B lights having respective wavelength bands in a time-sharing manner (refer to, for example, Japanese Patent Application Laid-Open No. H06-347639).
FIGS. 10A and 10B are respectively front and side views of a typical color wheel assembly incorporating such a color wheel. Referring to FIG. 10B, a color wheel assembly 200 comprises a color wheel 100, a hub 105, and a motor 106. The color wheel 100 is a tricolor color wheel composed of a disk-like substrate 101 which is made of a light-transmitting material, for example, optical glass, and three filter sectors 102, 103 and 104 which are formed on a surface of the substrate 101, and which transmit exclusively, for example, R, G and B lights, respectively. The color wheel 100 thus structured is fixedly attached to the motor 106 via the hub 105 coaxially therewith. The color wheel assembly 200 operates such that the color wheel 100 is rotated by the motor 106 so that the filter sectors (R, G and B) 102, 103 and 104 sequentially have white light S falling incident thereon whereby the white light S is sequentially dispersed into R, G and B lights.
Another known method of the time-sharing color composition as described above is that, for example, Y (yellow) light which results from white light having only B light extinguished, and M (magenta) light which results from white light having only G light extinguished are sequentially dispersed by an optical system respectively into R and G lights, and into R and B lights, then the R and G lights are modulated into R and G images in parallel to be composed into a Y image while the R and B lights are modulated into R and B images in parallel to be composed into an M image, and the Y image and the M image are sequentially projected on a screen, or the like. Though two light valve elements, one for R light and the other for G and B lights, are required in this method, R light emitted from a white light source can be fully utilized thereby improving the brightness on the display. In this method, a color wheel assembly employs a color wheel which includes two kinds/colors of filter sectors to filter the Y and M colors, respectively.
Such a bicolor color wheel is shown in FIGS. 11A and 11B, which show its plan view and its cross-sectional view taken along a line A-A′. A color wheel 110 shown in FIGS. 11A and 11B is structured such that a disk-like substrate 111, which is made of a light-transmitting material, for example, optical glass, has two kinds of filter sectors 112 and 113 formed thereon, and such that, for example, each filter sector 112 is a Y transmitting filter to reflect R light only and to transmit other lights, and each filter sector 113 is an M transmitting filter to transmit B light only and to transmit other lights. In the remaining part of the description of the related art, a bicolor color wheel is exemplified, but the present invention is not limited thereto as win be known from the descriptions to follow later on.
The filter sectors 102 and 103 are usually constituted by an optical interference filter that is formed of a dielectric multi-layer film structured such that a dielectric thin film formed of a material having a high refractive index (e.g., TiO2, ZrO2, and ZnS), and a dielectric thin film formed of a material having a low refractive index (e.g., SiO2, and MgF2) are alternately laminated by an evaporation method, a sputtering method, or the like. The optical interference filter is superior in durability (heat resistance, light stability, and chemical resistance) to a color filter formed by a staining method, a pigment dispersion method, or the like, has a high transmittance, and readily achieves a sharp spectroscopic characteristic, and therefore endures exposure to intensive light flux and produces a display image of a high visual quality.
Filter sectors, which are formed of the aforementioned dielectric multi-layer film, are often demarcated by means of a masking jig of a metallic thin plate (hereinafter referred to as “metal mask”). A method of forming the filter sectors accomplished by using metal masks are explained in FIGS. 12A to 12D, in each of which a plan view of the bicolor color wheel 110 under fabrication is shown on the left, and a cross-sectional view thereof taken along a line A-A′ is shown on the right. FIG. 12A shows the color wheel 110 at a process, where a metal mask 210 having openings 212 corresponding to the filter sectors 112 is duly positioned and fixedly set on one side surface of the substrate 111, FIG. 12B shows the color wheel 110 at a process, where the filter sectors 112 are formed by an evaporation method, a sputtering method, or the like, FIG. 12C shows the color wheel 110 at a process, where a metal mask 220 having openings 213 corresponding to the filter sectors 113 is duly positioned and fixedly set on the one side of the substrate 111, and FIG. 12D shows the color wheel at a process, where the filter sectors 113 are formed by an evaporation method, a sputtering method, or the like. The method using metal masks is more cost-effective and environmentally-friendly than, for example, a photolithographic method, or the like.
Different kinds of filter sectors corresponding to different colors and adjacent to each other are required to abut each other precisely and tightly unless achromatic areas which do not constitute any filter sectors are intentionally disposed. This is because if the adjacent filter sectors do not abut each other precisely and tightly, a gap is generated between the adjacent filter sectors, and light passing the gap fails to definitely determine its color thus resulting in not fully contributing to forming an image. Accordingly, in the above-described method of forming the filter sectors, it is crucial to precisely position and set, on the substrate 111 where the filter sectors 112 are already formed, the metal mask 220 for forming the filter sectors 113. To this end, at the aforementioned process shown in FIG. 12C, the metal mask 220 is first guided mechanically, for example, with a positioning pin, and then finally lined up by viewing, for example, through a microscope, the peripheries of the filter sectors 112 already formed and sidewalls W′ defining the opening 213 of the metal mask 220.
However, the following problem is found in the positioning technique described above. Since s metal mask usually has a thickness of about 100 μm, it occasionally happens at the process of forming the filter sectors 112 that as shown in FIG. 12B, dielectric multi-layer films constituting the filter sectors 112 fail to achieve a predetermined thickness at regions D which extend along sidewalls W defining the openings 212 of the metal mask 210, and which measure several 10 to 100 μm in width. In such a case, the color of the film changes continuously at the regions D to become achromatic making it difficult to clearly determine the demarcation of the filter sectors 112 even by viewing through a microscope, and this hinders precise alignment of the sidewalls of the openings 212 to the filter sectors 112. Consequently, the filter sectors 112 and the filter sectors 113 are positioned with respect to each other with a lowered degree of accuracy, and incomplete filter portions C are inevitably found at boundaries B between the filter sectors 112 and the filter sectors 113 (refer back to FIG. 11B).
In order to overcome the problem, for example, Japanese Patent Application Laid-Open No. H11-222664 discloses a metal mask with openings, in which the sidewalls of the openings are inclined with respect to the metal mask surfaces such that the openings have an increased area at one of the surfaces facing an evaporation source so that particles from the evaporation source come into the openings with reduced restriction thereby better achieving uniform film formation within the openings.
The aforementioned Japanese Patent Application Laid-Open No. H06-347639 discloses that filter sectors are desired to abut each other unless achromatic areas which do not constitute any filter sectors are intentionally disposed, but does not teach how it can be achieved. Also, the aforementioned Japanese Patent Application Laid-Open No. H11-222664 discloses a method that is anticipated to be good to a certain degree for clearly demarcating a boundary between filter sectors provided that an optimum inclination angle of the sidewalls surely exists and can be obtained somehow, which allows a film to be formed uniform in thickness all the way up to the peripheries of filter sectors. The optimum inclination angle of the sidewalls, however, must be obtained theoretically and experimentally based on various considerations, such as a film material, method and conditions of film formation, a desired film thickness, a metal mask thickness, and the like, and therefore the method disclosed therein cannot be readily applied to fabrication of a color wheel.